1. Field of the Invention
The present invention relates to a moisture sensitive resistive element and, more particularly, to an improved moisture sensitive resistive element of thin or thick film type which is sensitive to change of moisture content, or humidity, in the atmosphere in wide range covering 0% to 100% of relative humidity and has a quick response to such a humidity change to vary its internal ohmic resistance, a high reproducibility of moisture sensitive characteristic, and a high stability for a long period of time.
2. Description of the Prior Art
The moisture sensitive resistive element is an electric element which changes its internal ohmic resistance relative to the moisture content, or humidity, change in the atmosphere. According to the prior art, the moisture sensitive resistive element is formed by, for example: (1) a metallic oxide film or sintered metallic oxide material, such as iron oxide (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4) or tin dioxide (SnO.sub.2); (2) a material containing electrolyte salt, such as lithium chloride (LiCl); (3) a moisture absorbent, or hygroscopic, resin or high polymer film in which fiber or fine particles of electric conductive material, such as carbon particles are dispersed; (4) a device utilizing a temperature sensitive element, such as thermistor; or (5) a hydrophilic polymer film.
In general, a moisture sensitive resistive element utilizing a metallic oxide has a high heat resistance and a high response to humidity change, but has such a disadvantage that, particularly in the case of sintered metallic oxide material, the moisture sensitive characteristic is highly dependent upon structural factors, such a surface area, the density of sintered material, and the size of a metallic oxide particle, and, therefore, the sintered metallic oxide material has a poor interchangeability and reproducibility of the moisture sensitive characteristic.
As to the moisture sensitive resistive element containing electrolyte salt, such as lithium chloride, a range of humidity which a single moisture sensitive resistive element can detect is narrow and, therefore, it is necessary to provide two or more elements sensitive to different humidity ranges to enable the moisture detection over an entire range covering from 0% to 100% of relative humidity. Furthermore, when the moisture sensitive resistive element of this type is disposed in a high humidity atmosphere, such as in an atmosphere having 90% to 95% of relative humidity, for a long period of time, there may occur an eluation or dilution of electrolyte salt and, as a result, the moisture sensitive characteristic becomes very poor and shortens the life of the element.
As to the moisture sensitive resistive element comprising electric conductive particles or fibers dispersed in a moisture absorbent resin, the degree of resistance change is very sharp under a high humid atmosphere, but it is very dull under a low humid atmosphere. Although this type of element is suitable for use in a dew detecting device, it is not suitable for detecting a moisture in a wide humidity range.
As to the moisture sensitive resistive element utilizing a temperature sensitive element, such as a thermistor, the moisture content in the atmosphere is detected indirectly by the temperature change of a self-heated thermistor, utilizing a fact that the heat conductivity of gas or air changes dependently on the amount of water vapor contained therein. This type of moisture sensitive resistive element may be used for detecting an absolute humidity but has such a disadvantage that the element is susceptible to surrounding temperature and wind.
As to the moisture sensitive resistive element using a hydrophilic polymer film, there are many advantages. For example: the humidity range which the element can detect is wide; the response to the humidity change is very fast; the structure of the element is rather simple; and the element can readily be manufactured at low cost. This type of element, however, has a poor resistance to moisture and water and, therefore, its life is very short.